If I may interject, overriding Kobo's firmware with KOReader (simple drag-n-drop of a file) gave a second life to my Aura 2. It's made it responsive and snappy while providing more features. It may not be necessary for newer ones (or for cover art on sleep), but I was on the fence to upgrade my 5 year old e-reader before trying it, feels like a new device now.
https://www.kobo.com/us/en/search?query=standard%20ebooks&ac...
In any case almost nobody is buying these anyway, as there are so many other free ebook editions of just about all of these books already.
I'm surprised Kobo doesn't have a list of banned seller names for situations like this. It would take zero effort on their end, and they're clearly opening themselves to liability.
Off to create Amazon and Disney stores over there... /s
I use a Kobo eink device myself and haven't noticed the cover art problem you're describing. Make sure you're on the latest firmware, and that you're transferring our kepub files using a USB cable and not Calibre. (Calibre may attempt to apply their own conversion on top of our own conversion, which can result in unexpected things happening.)